difference in algorithms of both search engines. Google and Yahoo is follows own individual algorithms. Bzc, your rankings are different.
http://help.yahoo.com/l/us/yahoo/search/webcrawler/slurp-12.html Note: Slurp may use a "nofollow" link for discovering content, but the link will not be considered an "approved" link for consideration for ranking of the target page.
From my experience Google look at both content and backward links, whereas yahoo look at more content. If you have optimised your content to suit Google then it may not be 'spammy' enough for Yahoo, or it might be to weak. There can be a correct balance, where you do come up well for both, but personally i concentrate on getting top in Google as it dominates the search world.
Not Specially Changes of different search engine to different change all the search engine algorithm is different you more focus in your off pages work